Two human retroviruses, human immunodeficiency virus type 1 (HIV-1), the cause of acquired immune deficiency syndrome, and human T-cell leukemia virus type 1 (HTLV-1), which causes adult T-cell leukemias and lymphomas, are responsible for significant mortality and morbidity worldwide. The objective of this exploratory project is to determine whether a relatively unstudied aspect of retroviruses, their propensity to non-randomly recruit host noncoding RNAs into virions, can be exploited to gain insights into retroviral biology, to identify potential new targets for antivirals, and to engineer Trojan RNA variants that, upon packaging, interfere with essential processes. The specific focus is on a new human retrovirus, called Xenotropic murine leukemia virus-related virus (XMRV), which was recently identified in some prostate tumors and in some patients with chronic fatigue syndrome. Although it is not yet established whether XMRV contributes to the pathogenesis of either disease, the finding that humans harbor a novel infectious retrovirus is nonetheless a significant cause for concern. Although noncoding RNAs have been known to undergo packaging by retroviruses for forty years, the spectrum of RNAs encapsidated is unknown, their roles in retrovirus biology are poorly understood, and their potential as antiviral targets has not been explored. We chose XMRV for these experiments because of its potential link to two diseases and for its close homology to MoMuLV, the retrovirus in which host RNA encapsidation has been best studied. The first aim is to use deep sequencing technology to elucidate the complete set of host RNAs packaged by XMRV. The second aim is to determine the molecular basis and functional relevance of host RNA packaging by XMRV. For those RNAs that we find to be stoichiometric components of virions, we will identify the determinants within both the virus and the RNA that confer packaging. We will also test if preventing packaging of these host RNAs affects virus assembly or function. Depending on the particular RNA, this will involve either depleting the RNA directly with siRNAs or depleting protein partners that stabilize the RNA. For those host RNAs that are packaged as newly synthesized transcripts, we will also try to decrease packaging by overexpressing partner proteins to drive more of the RNAs into the mature RNPs. The third aim is to develop virotoxic derivatives of highly packaged host RNAs. For each potential virotoxic derivative, we will assess its expression in host cells, its packaging by XMRV, and whether the presence of the RNA affects virus yield or infectivity. Together, these experiments should allow us to elucidate the set of host noncoding RNAs that contribute to the XMRV lifecycle and may provide paradigms for designing new classes of antiviral agents that target both XMRV and known pathogenic retroviruses such as HIV-1 and HTLV-1. PUBLIC HEALTH RELEVANCE: Retroviruses such as HIV-1 cause serious illnesses, such as AIDS, that affect millions of people worldwide. A new retrovirus, called XMRV, was identified in some prostate tumors and in some people with chronic fatigue syndrome. This proposal tests whether the tendency of XMRV and other retroviruses to capture RNA molecules from infected cells can be used to identify new targets for designing antiviral drugs.